Bragg reflectors make it possible to reflect a portion of the light that they receive. They are commonly used in the field of optical transmission. They can be disposed facing an integrated laser cavity so as to return energy to thereto and so as to enable it to oscillate continuously. In which case, they replace the cleaved-face mirrors of non-integrated semiconductor lasers.
A Bragg reflector is more particularly defined by a periodic grating made up of two materials having different refractive indices. The reflection ratio of such a device depends both on the difference between the indices of the two component materials, and also on the geometrical shape of the grating. It is proportional to the product of .kappa. multiplied by L, where L is the length of the grating, and .kappa. is a coupling coefficient relating both to the difference in the indices of the two materials, and also to the thickness of the grating.
A conventional reflector is shown in longitudinal section in FIG. 1A and in cross-section in FIG. 1B. That reflector is formed on layers 2, 3 stacked by epitaxy on a substrate 1. In general, the substrate 1 is made of indium phosphide (InP) doped with n-type carriers. The layers of the stack perform different optical functions. In the example shown in FIGS. 1A and 1B, an active layer 2, also referred to as the "waveguide", is deposited on the substrate 1. The active layer 2 is buried in a "bottom cladding" layer 3 made of a material of the III-V type, such as InP.
The Bragg grating is referenced 10 in FIG. 1A. It is made up both of a material 4 based on InP, such as, for example, a quaternary material of the GaInAsP type, represented by hatching in FIGS. 1A and 1B, and of an InP material 5 constituting a top cladding layer.
The grating is formed by epitaxially growing a layer 4 of the quaternary material on the bottom cladding layer, by etching the layer 4 to form a crenelated pattern having a period .LAMBDA., then by filling in the resulting notches with a top cladding layer 5 of InP doped with carriers. The quaternary material GaInAsP 4 and the InP 5 have different refractive indices which are equal respectively to 3.3 and to 3, and the resulting holographic grating 10 reflects part of the light wave that it receives.
However, as shown in FIG. 2, in order for the spectrum window of that type of grating to be wide enough, i.e. in order to make it possible for the grating to be used for a large number of wavelengths, its coupling coefficient .kappa. must be very high. Thus, as shown by the curves of FIG. 2 which correspond to a conventional grating that is 35 .mu.m long, in order to obtain a spectrum width of about 20 nm, the coupling coefficient .kappa. must be about 500 cm.sup.-1, and in order to obtain a spectrum width of about 30 nm, the coupling coefficient .kappa. must be about 1000 cm.sup.-1. Unfortunately, in order to obtain a high coupling coefficient, it is necessary to form a grating that has a large index step and/or large width.
To form such a grating having a high coupling coefficient, a first known solution consists in increasing the thickness of the grating in a manner such that the light wave perceives contrast that is as great as possible. The contrast is related both to the type of the material and to the thickness of the grating, i.e. to the thickness of the layer of quaternary material. Unfortunately, that solution does not, in practice, make it possible to form a reflector that has the desired performance. If the notches formed in the quaternary material are too deep, the filling-in step becomes difficult to perform without degrading the crystal quality of the filling-in material, i.e. of the top cladding layer. Unfortunately, when the crystal quality of said top cladding layer is affected, light propagation losses occur, so that the performance of the device is reduced, and the Bragg coefficient .kappa. does not increase sufficiently to obtain a wide spectrum window.
Another solution has been considered for increasing the index step. That solution consists in etching a holographic grating, obtained conventionally between a quaternary (or ternary) material 4 and a cladding layer is of InP 5, so as to remove the quaternary (or ternary) material selectively, and to form another grating structure having a large index step between air and the InP semiconductor. In which case, the value of the index step is large: about 2. Unfortunately, even though it has a wide spectrum window, that type of grating suffers from numerous interface coupling losses when it is coupled to the waveguide of another device, such as a laser device, for example.
It is therefore difficult to increase the coupling coefficient .kappa. of a holographic Bragg grating without degrading its performance.